A model for the frequency fine structure of auroral kilometric radiation

Abstract

One of the more interesting and as yet unexplained features of auroral kilometric radiation (AKR) is the frequency fine structure observed in wideband spectrograms. This fine structure consists of emission features that drift in frequency with time, and of banded emission structure. Both positive and negative drifts are observed. While the electron cyclotron maser instability is widely believed to be the source of AKR, no proposed explanation for the frequency fine structure seen in AKR has been confirmed. In this paper, we propose a model for AKR frequency fine structure in which the fine structure is a natural consequence of the emission of electron cyclotron maser radiation in a nonuniform magnetic field. This model is examined by using one‐dimensional, electromagnetic, particle‐in‐cell simulations. We find that maser radiation is emitted as individual wave packets that combine to form the drifting emission features that make up the fine structure. Rising frequency drifts are associated with wave packets emitted independently of one another. Wave packets that interact with one another produce features with falling frequency drift. Combinations of these wave packets can produce features that rise and fall. Both X mode radiation and O mode radiation produce fine structure with narrowband, drifting emissions. The emission features produced have a large spread in both positive and negative frequency drift rates. We find that the average positive drift rate of rapidly drifting features is approximately proportional to the maser growth rate. On the other hand, the average negative drift rate of rapidly drifting features remains approximately constant. For slowly drifting features, specifically, features with drifts that may be close to observed drift rates, both the positive and negative drift rates are approximately proportional to the magnetic field gradient.